Reusable medical instruments and pharmaceutical and biological equipment are generally sterilized before each use. Additionally, reusable containers employed in medical, pharmaceutical, and biological applications, such as glove boxes and incubators, are generally sterilized before each use. In facilities and applications where these types of instruments and containers are used several times a day, it is important to achieve sterilization efficiently and economically. Food packaging materials, such as soda bottles, milk containers, and the like, are also microbially decontaminated prior to filling. At current bottling plant flow rates of a thousand bottles/hour or more, it is desirable to optimize sterilization conditions for rapid sterilization.
Several different methods have been developed for delivering a vapor phase sterilant to an enclosure or chamber for sterilizing the load (e.g., medical instruments or other articles) or interior thereof. In one option, the “deep vacuum” approach, a deep vacuum is used to pull liquid sterilant into a heated vaporizer. Once vaporized, the sterilant vapor is drawn into an evacuated and sealed chamber. In another option, the “flow-through” approach, vaporized sterilant is mixed with a flow of carrier gas that serves to deliver the sterilant vapor into, through and out of the chamber, which may be at slightly negative or positive pressure.
Methods have been developed for optimizing a vapor phase sterilization in a deep vacuum and/or flow-through system. Cummings, et al., U.S. Pat. No. 4,956,145, discloses a deep vacuum method of vapor phase sterilization in which a predetermined concentration of hydrogen peroxide sterilant vapor is maintained in an evacuated, sealed chamber. The amount of liquid sterilant injected into a vaporizer is regulated or adjusted to account for the estimated decomposition of hydrogen peroxide sterilant vapor into water and oxygen in the closed system over time. In a different approach, a predetermined percent saturation is maintained in an open, flow-through sterilization system as disclosed in U.S. Pat. No. 5,445,792.
Childers, U.S. Pat. No. 5,173,258, discloses another flow-through system in which vapor phase hydrogen peroxide is introduced into a recirculating, closed-loop flow of carrier gas. The hydrogen peroxide is introduced and maintained at a predetermined concentration which is selected to optimize the sterilization cycle. The system includes a dryer to dehumidify the recirculating flow, preferably to at least about 10% relative humidity, and thereby prevent moisture buildup resulting from the decomposition of hydrogen peroxide vapor over time. By eliminating moisture build-up, the system can maintain the sterilization chamber at higher concentrations of vapor phase hydrogen peroxide sterilant for longer periods of time (i.e., the predried gas will accept more of the sterilant vapor). Further, to avoid condensation of the sterilant, the relative humidity in the chamber is preferably reduced (e.g., to less than about 30%) prior to introducing the sterilant vapor. After decontamination is complete, the enclosure may be re-dehumidified or conditioned if desired for the selected application.
Gaseous and vapor sterilization/decontamination systems rely on maintaining certain process parameters in order to achieve a target sterility or decontamination assurance level. For hydrogen peroxide vapor sterilization/decontamination systems, those parameters include the concentration of the hydrogen peroxide vapor, the degree of saturation, the temperature and pressure, and the exposure time. By controlling these parameters, desired sterility assurance levels can be successfully obtained while avoiding condensation due to vapor saturation. Existing systems typically monitor the amount of liquid delivered to the vaporization system over time, and, based on temperature, pressure, volume, and (where applicable) flow rate, calculate the theoretical concentration of hydrogen peroxide vapor. The system then correlates some or all of these parameters with empirically derived estimates of hydrogen peroxide decomposition, to arrive at an estimate of the amount of hydrogen peroxide to inject into the system in order to maintain a selected theoretical concentration of hydrogen peroxide vapor. The sterilization performance is then validated empirically via microbiological efficacy testing.
In actual practice, several factors can affect the concentration of components of the vapor, such as decomposition, absorption and adsorption, all due to contact of the gas with various surfaces in the system, and dilution due to evaporation by water vapor from the loads being processed and to decomposition of the sterilant. These effects can vary from load to load and system to system. It is, therefore, desirable to measure the hydrogen peroxide concentration directly. Stewart, et al., U.S. Pat. No. 5,872,359, discloses a hydrogen peroxide sensor system and method of control of a vapor sterilization chamber. The sensor uses near infra-red (NIR) detection at two specific wavelengths, one corresponding to a predominantly hydrogen peroxide peak, the other to a water peak. There is some overlap between the peroxide and water peaks. By manipulating the data, the contribution of water can be subtracted out and the hydrogen peroxide concentration determined.
The present invention provides a new and improved hydrogen peroxide vapor sensor and method of use, which overcomes the above-referenced problems and others.